Re: [PATCH] qemu-nbd: Support -s and -l at the same time

[Eric Blake <eblake@redhat.com>]

[checking through my old branches]

On Wed, Apr 08, 2026 at 11:03:31AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 08.04.26 05:16, Eric Blake wrote:
> > qemu-nbd already has the ability to wrap a read-write layer on top of
> > the export of a read-only image (-s) and the ability to export an
> > internal snapshot rather than the main image in a qcow2 file (-l
> > SNAPID).  But it gave a confusing error message when trying to mix the
> > two, because the snapshot overlay created first no longer has access
> > to the internal snapshots of the image being wrapped.
> > 
> > WIth a bit of finesse, this useful scenario is now supported.
> > 
> > https://gitlab.com/qemu-project/qemu/-/work_items/159
> > 
> > Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> 
> Some optional suggestions below:
> 
> > +
> > +echo
> > +echo "=== Main image in snapshot mode ==="
> > +echo
> > +
> > +nbd_server_start_unix_socket -s -f qcow2 "$TEST_IMG"
> > +$QEMU_IO -c 'r -P 2 3M 1M' -c 'w -P 3 3M 1M' -f raw \
> 
> Maybe, add read command after write, to check write correctness. [*]

Good idea.

> 
> > +  nbd:unix:$nbd_unix_socket | _filter_qemu_io
> > +nbd_server_stop
> > +
> > +echo
> > +echo "=== Read-only snapshot access ==="
> > +echo
> > +
> > +nbd_server_start_unix_socket -l snap -f qcow2 "$TEST_IMG"
> > +$QEMU_IO -r -c 'r -P 1 3M 1M' -f raw \
> 
> Suggest, add write command, to check that it fails.

qemu-io rejects attempts to do -c 'w ...' on a read-only connection,
but it is a good idea to prove that qemu-io won't connect read-write
to a read-only server.

I'm queueing this patch with the following squashed in.

diff --git i/tests/qemu-iotests/tests/qemu-nbd-snapshots w/tests/qemu-iotests/tests/qemu-nbd-snapshots
index de405e43720..5bc7a1a5d23 100755
--- i/tests/qemu-iotests/tests/qemu-nbd-snapshots
+++ w/tests/qemu-iotests/tests/qemu-nbd-snapshots
@@ -45,7 +45,7 @@ echo "=== Main image in snapshot mode ==="
 echo

 nbd_server_start_unix_socket -s -f qcow2 "$TEST_IMG"
-$QEMU_IO -c 'r -P 2 3M 1M' -c 'w -P 3 3M 1M' -f raw \
+$QEMU_IO -c 'r -P 2 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \
   nbd:unix:$nbd_unix_socket | _filter_qemu_io
 nbd_server_stop

@@ -56,6 +56,9 @@ echo
 nbd_server_start_unix_socket -l snap -f qcow2 "$TEST_IMG"
 $QEMU_IO -r -c 'r -P 1 3M 1M' -f raw \
   nbd:unix:$nbd_unix_socket | _filter_qemu_io
+$QEMU_IO -c 'r -P 1 3M 1M' -f raw \
+  nbd:unix:$nbd_unix_socket 2>&1 | _filter_qemu_io | _filter_testdir
+
 nbd_server_stop

 echo
@@ -63,7 +66,7 @@ echo "=== Combine -s and -l for read-write internal snapshot access ==="
 echo

 nbd_server_start_unix_socket -s -l snap -f qcow2 "$TEST_IMG"
-$QEMU_IO -c 'r -P 1 3M 1M' -c 'w -P 3 3M 1M' -f raw \
+$QEMU_IO -c 'r -P 1 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \
   nbd:unix:$nbd_unix_socket | _filter_qemu_io
 nbd_server_stop

diff --git i/tests/qemu-iotests/tests/qemu-nbd-snapshots.out w/tests/qemu-iotests/tests/qemu-nbd-snapshots.out
index 6bce9d90253..47289c30f1d 100644
--- i/tests/qemu-iotests/tests/qemu-nbd-snapshots.out
+++ w/tests/qemu-iotests/tests/qemu-nbd-snapshots.out
@@ -14,11 +14,14 @@ read 1048576/1048576 bytes at offset 3145728
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 1048576/1048576 bytes at offset 3145728
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1048576/1048576 bytes at offset 3145728
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

 === Read-only snapshot access ===

 read 1048576/1048576 bytes at offset 3145728
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-io: can't open device nbd:unix:SOCK_DIR/qemu-nbd.sock: Could not open image: Permission denied

 === Combine -s and -l for read-write internal snapshot access ===

@@ -26,6 +29,8 @@ read 1048576/1048576 bytes at offset 3145728
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 1048576/1048576 bytes at offset 3145728
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1048576/1048576 bytes at offset 3145728
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

 === Check that original image is unchanged ===


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org